Nitrogen oxides (NOx) are among the primary air pollutants emitted by combustion processes. Because nitrogen oxides promote the formation of harmful atmospheric reaction products that cause smog, air quality standards have been imposed by various government agencies to limit the amount of NOx that can be emitted into the atmosphere. As a result of the increasing environmental legislation in many countries and increasing global awareness of atmospheric pollution, modern combustion technology has been improved to curb NOx emissions from many types of combustion equipment. Most of the efforts to date have focused on the reduction of thermal NOx, which contributes the largest portion of NOx formed by combustion. Many of the conventional NOx reduction techniques such as staging suppress thermal NOx formation but are not effective against prompt NOx. Advances in combustion technology are needed to control the generation of both thermal NOx and prompt NOx to reduce total NOx emission levels to below 10 ppmv with air fuel firing.
The secondary metals industry is generally considered to be a major source of NOx pollution and therefore is subject to stringent regulations on NOx emissions. The reduction of NOx production in combustion processes becomes more important in this industry as the demand for metals increases while environmental regulations on NOx become increasingly stringent. Full oxy-fuel combustion theoretically can produce very low NOx emissions due to the lack of nitrogen in the oxidant. However, certain industrial processes such as secondary aluminum melting are susceptible to large amounts of air ingress into furnaces in which the extremely high flame temperatures of an oxy-fuel flame increase thermal NOx formation. The steel industry also is facing similar challenges in the control of NOx emissions, and processes such as steel reheating and ladle preheating are large NOx generators due to air leakage into the process furnaces.
Air-fuel combustion is notoriously inefficient without heat recovery methods. Using oxygen or oxygen-enriched air in place of air for combustion increases the flame temperature and thus the radiative heat transfer to the load, and also greatly increases the amount of available process heat from the combustion process by eliminating the wasteful heating of the nitrogen in air. Air-fuel firing with heat recovery, the best example of which uses regenerative burners, is an alternative to oxy-fuel firing. In a common configuration, regenerative burners are installed in pairs in a furnace such that one burner fires while the other burner serves as an exhaust conduit through which the flue gas exits the furnace and flows through a bed of heat storage material. At preset intervals, the two burners switch roles, and the combustion air is preheated by flowing over the regenerative bed. Most of these regenerative air-fuel burners usually produce NOx emissions in the several hundred ppmv range.
A relatively new technique of NOx control is described in the art as flameless combustion wherein the reactants for combustion are highly diluted before they mix and react. The reactants usually are diluted by entraining combustion products before the combustion reactions occur. This mode of combustion typically occurs when the oxidizing gas is diluted to a level below 17% oxygen, wherein the flame front disappears and the fuel oxidizes in a flameless fashion. The key to this technology is the maintenance of the furnace temperature above the auto-ignition temperature of the fuel and the use of a highly-robust flame stabilizer.
There is a need in the process industries, and particularly in the secondary metals industries, for improved combustion technology that reduces NOx formation and optimizes energy consumption. Improvements to flameless combustion processes are needed for burners that can fire in various modes using various oxidants including air, enriched air, pure oxygen, and/or combinations thereof while achieving low NOx production during all modes of operation. There also is a need for improved flameless combustion processes that effect uniform heat distribution to the raw metal charge in melting furnaces.
These needs are addressed by the embodiments of the present invention as described below and defined by the claims that follow.